So out of boredom I modified my Game Genie so could work in a Japanese Mega Drive. After doing this I remembered reading about how certain early release Genesis games also had the Japanese version still on the cart. I've only checked a few of my games but here's the ones that I've checked so far.
If anybody else has done this and wants to add games to the list please post about them.
Bio Ship Paladin
Bonanza Brothers
Columns
Cyberball
ESWAT: City Under Siege
Fatal Labyrinth
Final Zone
Fire Shark
Flicky
Forgotten Worlds
Gain Ground(JP)
Ghostbusters
Ghouls N Ghosts
Grind Stormer
Mercs
Midnight Resistance
Mystic Defender
Quackshot
Revenge of Shinobi
Streets of Rage
Streets of Rage 2
Thunder Force 2
Truxton
Twin Hawk